This invention relates to polyimide resins prepared from aromatic diamines and phenylethynyl phthalic anhydride or phenylethynyl-substituted phthalic anhydrides. These polyimide resins have very low melt viscosities and, when cured, have good mechanical properties and are stable at very high temperatures.
Certain aromatic polyimides are known for their exceptional thermal, thermooxidative, and chemical resistance. These polymers are typically prepared by the polymerization of aromatic diamines and dianhydrides in a 2-step synthesis, first to form a poly(amide acid) precursor, which is then cyclodehydrated to the corresponding polyimide, as described in Hergenrother and Smith, "Chemistry of Imide Oligomers End-capped with Phenylethynylphthalic Anhydrides", Vol. 35, Polymer, p. 4857 (1994). However, such resins are difficult to melt process due to their high melting temperatures and viscosities which restrict their use in structural adhesive and composite applications. The melt processability of such oligomers or polymers may be altered, for example, by the incorporation of flexible segments or by making low molecular weight oligomers which are end-capped with reactive groups. However, the viscosities of these different types of oligomers and polymers may still be relatively high.
If an end-capping compound is used, such compound typically also contains a latent-curable reactive group in addition to the group which caps the oligomer or polymer, which reacts with other end groups in the resin at high temperatures. An article may be prepared by, for example, processing the uncured resin into a desired shape, and then heating the resin to cause the end groups to react and cure the resin. The use of end-capping agents with latent-curable groups provides a means to chain-extend or crosslink the polymer at high temperatures, which increases the Tg of the cured polymer. One type of end group that has been used for this purpose is the phenylethynyl end group, as described in U.S. Pat. Nos. 5,599,993 and 5,493,002. Another approach has been to locate the phenylethynyl groups pendant to the oligomer backbone, by means of a trifunctional aryl compound having two diamine groups and one phenylethynyl group, as disclosed in U.S. Pat. No. 5,689,004. These pendant end groups have been found to be stable under the polyimide synthesis conditions, and at temperatures of up to 250.degree. C. Such pendant groups will then react and cure at temperatures above about 300.degree. C., resulting in a polymer with a higher Tg than that of a resin that was not prepared using the phenylethynyl pendant groups. Phenylethynyl phthalic anhydride end-capping reagents are described in U.S. Pat. No. 5,567,800. However, the melting temperatures and the melt viscosities of the above-described oligomers having pendant or terminal phenylethynyl groups may be too high for certain methods of processing. Another approach to lowering the melt viscosity of the resin has been to add reactive diluents having two phenylethynyl end groups, as described in U.S. Pat. No. 5,426,234. However, reactive diluents may adversely affect the thermal and physical properties of the cured polymer, such as by reducing its Tg or toughness.
The low melt viscosities obtained for the compositions of this invention are particularly useful in the preparation of high performance composites via resin transfer molding. Resin transfer molding is a process that utilizes a preform, which consists of multiple plies of graphite, glass or other reinforcing fiber. This preform is placed in a mold, which is then filled with a thermosetting resin that infiltrates and wets the fibers of the preform. This process of consolidating the fiber and the resin may be aided by the use of pressure. After this step, the resin is then cured to produce a composite part. Composite parts produced by resin transfer molding are typically less expensive to make than those obtained by other methods, such as those using prepregs, due to the fact that resin transfer molding is not as labor intensive. Resin transfer molding requires the use of thermosetting resins which have a low melt viscosity to permit filling the mold, which can be of a complex shape, and to infiltrate and wet out the preform, which may be comprised of multiple layers of fibers. In addition to having a low viscosity, resins used in resin transfer molding need to have cured resin properties such that the parts produced are of a relatively high strength, stable at high temperatures and are not too brittle.